1. Technical Field
The present invention an improved apparatus for stirring polymer particles in reactive, polymerization reactors incorporating a hydraulic motor on a coaxial drive shaft within the reactor.
2. Description of the Prior Art
U.S. Pat. No. 3,639,377 (Trieschmann et al.) describes polymerization of propylene which is carried out in the gas phase. In order that heat of polymerization should be effectively removed, excess monomeric propylene is introduced in liquid or partly liquefied form into the bottom of a vertically disposed cylindrical reaction zone. During polymerization, the fraction of unpolymerized propylene evaporates while absorbing the heat of polymerization. Evaporated propylene is removed from the reaction zone and condensed again outside the reaction zone.
While stating that removal of heat according this system of internal cooling also causes intense mixing of the solid polymer powder with the gas phase, Trieschmann et al. state that it is particularly advantageous to use a reactor having a spiral stirrer and conventional electrical motor with gearbox. Referring to FIGS. 1 and 3 of U.S. Pat. No. 3,639,377, the stirrer appears to be illustrated as having a “U” shape and rotates on a vertical shaft through the bottom of vertical reactor 6. Catalyst is pumped into the reactor through the top and polymer is discharged from the reactor by means of an external cyclone.
U.S. Pat. No. 3,944,534 (Sennari et al.) describes gas-phase polymerization of an .alpha.-olefin which is carried out in a reaction bed formed by circulation of particulate olefin polymer, caused principally by mechanical agitation to undergo circulation in the up-and-down directions within a substantially vertical-cylinder type reactor. The single-stage back-mixing reactor type described in Sennari et al. is not suitable for use in a continuous polymerization process with high activity catalysts, because age of catalyst carried out of the reactor is substantially the same as the age of catalyst in a back-mixing reactor.
Vapor-phase polymerization of a polymerizable monomer or mixture thereof to produce normally solid polymer substances using a horizontal polymerization reactor containing a subfluidized particulate bed of polymerized monomer has been described in a number of patents including: U.S. Pat. No. 3,957,448 (Shepard et al.), U.S. Pat. No. 3,965,083 (Jezl et al.), U.S. Pat. No. 3,971,768 (Peters et al.), and U.S. Pat. No. 4,627,735 (Rose et al.), the disclosures of which are specifically incorporated herein in their entirety by reference. These references describe polymerization processes and apparatus in which polymer is formed from gaseous monomer in horizontal stirred bed vessels.
In a single reactor, polymerization of monomer or mixture thereof from the vapor state is carried out by an essentially isobaric process typically using a high yield catalyst and cocatalyst. Typically, in operation of such processes and apparatus, particles of polymer are formed around solid catalyst particles.
The horizontally or vertically disposed reactor vessel has recycle gas, such as propylene, introduced into the bottom thereof. Typically, quench liquid, such as liquid propylene, is injected into the reactor from the top of the reactor, but may also be introduced with the circulating, optionally fluidizing, gas or directly into the polymerization bed.
Gases and vapors within the reactor vessel are free to circulate and mix together throughout the vapor space. For continuous production of some polymers, particularly copolymers, where it may be necessary to have different gas compositions at subsequent stages of polymerization, a series of two or more reactors is required.
Paddle wheels or other types of stirring vanes inside the vessel sweep through the bed of polymer particles and stir the contents of the vessel. The various types of stirring vanes include staggered paddles, inclined paddles, spiral vanes, or vanes provided with a scraper for scraping the internal wall of the reactor vessel.
A solid transition metal-containing catalyst component is injected at least one point into the reactor, and an aluminum alkyl cocatalyst plus modifiers may be injected at adjacent points.
When using a horizontal sub-fluidized bed reactor the catalyst is preferably added into the top, near one end (front end disposed opposite to a take-off end), of the reactor vessel. Solid particles of polymerized monomer are created in the vessel and are withdrawn from the take-off end thereof. Particles of polymerized monomer build up in the stirred reactor and traverse the length of the reactor essentially because of polymerization in the bed and not by the agitator. Advantageously, this condition is ensured by the design of the agitator such as to provide for agitation, but not for significant backward or forward movement of the particles. Since this stirred bed is not in a fluidized condition, back-mixing of the particles of polymerized monomer in the horizontally disposed reactor vessel is limited.
In contrast, solid particles in a fluidized bed are very well mixed. Even at commercially useful ratios of length to diameter, horizontal stirred-bed reactor systems can readily achieve a degree of mixing of solids equivalent to two, three, or more theoretical back-mix reactors. Thus, horizontal stirred-bed reactor systems are particularly advantageous, as compared fluidized-bed reactors, for direct production of polymers in a particulate no form.
This invention is however equally applicable to stirred vertical sub-fluidized or fluidized bed reactors. The bed may be kept in a fluidized state by introducing gaseous components, e.g. monomer on such flow rate (at least 0.2 m/s) which make the particles act as a fluid. The gas phase reactor used can be operated in the temperature range of 50 to 115 degrees centigrade, preferably between 60 and 110 degrees centigrade and reaction pressure between 10 and 40 bar and below the dew point. The partial pressure of the monomer is preferably between 2 and 40 bar or more. The polymerization bed is kept well mixed through use of agitator blades, preferably upward vertically extending paddles, connected to agitator arms attached to the drive shaft of a stirrer. The agitator arms may be horizontally extending arms with a freely selectable cross section. Typically the cross section of the arms is designed for minimum agitation resistance. The length of the agitator arms are usually designed such that the agitator blades connected to the ends of the agitator arms extend as close as possible to the inner walls of the reactor vessel where the fluidization action is inherently weakest. At least a portion of the gas flow introduced to the reactor may be passed to the reactor via a one flow channel provided to the inside of the agitator shaft. These reactors may be greater than 2 m in diameter, preferably greater than 4 m in diameter, most preferably greater than 5 m in diameter. The larger the reactor, the more difficult it is to maintain homogeneous fluidization and stirring throughout the entire volume of the fluidized bed.
It is desirable to create polymer particles as quickly as possible, and for this purpose a number of different high activity catalyst systems have been developed.
Use of solid, transition metal-based, olefin polymerization catalyst components is well known in the art including such solid components supported on a metal oxide, halide or other salt such as widely-described magnesium-containing, titanium halide based catalyst components. Such catalyst components commonly are referred to as “supported.”
As is well known in the art, particulate polymers and copolymers may be sticky, i.e., tend to agglomerate, due to their chemical or mechanical properties or pass through a sticky phase during the production cycle. Sticky polymers also are referred to as non-free flowing polymers because of their tendency to compact into aggregates of much larger size than the original particles and not flow out of the relatively small openings in the bottom of product discharge tanks or purge bins. Polymers of this type show acceptable fluidity in a gas phase fluidized bed reactor, however, once motion ceases, the additional mechanical force provided by the fluidizing gas passing through the distributor plate is insufficient to break up the aggregates which form and the bed will not refluidize.
Although polymers that are sticky can be produced in non-gas phase processes, there are certain difficulties associated with the production of such products in, for example, slurry or bulk monomer polymerization processes. In such processes, the diluent or solvent is present in the resins exiting the reaction system at a high concentration leading to severe resin purging problems particularly if the material in question is a low molecular weight resin or a very low crystallinity resin. Environmental considerations are such that the dissolved monomers and diluent must be removed from the polymer prior to its exposure to air. Safety also dictates the removal of residual hydrocarbons so that closed containers containing the polymers will not exceed safe levels for volatiles in the gas head space over the resin. The safety and environmental concerns are accompanied by a definite economic factor in determining a preference for a quench-cooled, vapor-phase polymerization reactor containing a subfluidized particulate bed of polymerized monomer. The low number of moving parts and the relative lack of complexity in a basic subfluidized bed process enhances the operability of the process and typically results in lower costs of production. Low costs of production are due, in part, to low volumes of recycled process streams and a high unit throughput.
Horizontal stirred-bed reactor systems disclosed in Shepard et al., Jezl et al., Peters et al., and in U.S. Pat. No. 4,101,289 ('289), U.S. Pat. No. 4,129,701 ('701), U.S. Pat. No. 4,535,134 (de Lorenzo et al.), U.S. Pat. No. 4,627,735 (Rose et al.), U.S. Pat. No. 4,640,963 (Kreider et al.), U.S. Pat. No. 4,883,847 (Leung et al.), U.S. Pat. No. 4,921,919, (Lin et al.) and U.S. Pat. No. 5,504,166 (Buchelli et al.), the disclosures of which are specifically incorporated herein in their entirety by reference, largely or completely solve problems related to vapor phase, solution or slurry polymerization and reaps important economic benefits through savings in energy consumption, raw materials, and capital costs.
Although previously-known vapor-phase polymerization systems are entirely satisfactory for manufacture of many commercial polymers, a need still exists for improved mechanical stirring in a quench-cooled subfluidized particulate bed of polymerized monomer during continuous vapor phase polymerization. Desirably, the improved process produces fewer lumps and strings of resin. Such lumps and strings tend to hang-up or become trapped in transfer equipment and can even plug lines and valves. More desirably, the improved transfer apparatus increases the range in physical properties of polymers which can be manufactured at high rates of production without interruptions in operation. Especially welcome are improved methods and apparatus which more closely achieve continuous steady-state conditions throughout the vapor-phase process and thereby produce polymer products having more uniform physical properties.
One problem with known polymerization processes and apparatus using a vapor-phase polymerization system, is that lumps and strings of resin can form in a quench-cooled subfluidized particulate bed of polymerized monomer without reliable, continuous and accurate methods for mechanical stirring. Polymers formed from alkenes of 2 to 8 carbon atoms such as propylene or a mixture of propylene and other lower alkenes often have a tendency to agglomerate under operating conditions during polymerization. Such sticky polymers are difficult to maintain in granular or particulate forms during polymerization, particularly where high rates of production are desired. Further, it is advantageous to maintain a uniform temperature profile along the reactor. Agitator apparatuses according to this invention advantageously are useful for stirring of polymer particles, particularly in subfluidized particulate beds of alpha-olefin polymers in high pressure, reactive gas-filled, continuous, vapor-phase polymerization reactors.